Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

Which condition would cause the altimeter to indicate a lower altitude than actually flown (true altitude)?

• A. Atmospheric pressure lower than standard.
• B. Air temperature warmer than standard.
• C. Air temperature lower than standard.
• D. Wind.

Answer: (D)

Temperature effects on altitude readings. The altimeter measures ambient pressure and converts it to an altitude. In warmer-than-standard air, the pressure falls off with height more slowly, so at a given true altitude the ambient pressure is higher than what the standard atmosphere would predict. With the altimeter set to the standard pressure lapse, this higher pressure makes the instrument indicate a lower altitude than you actually have. So the true altitude is higher than what the altimeter shows. Wind doesn’t change the pressure the altimeter senses in level flight; it mainly affects ground track and groundspeed. That’s why wind isn’t the cause of a lower indicated altitude. The warmer-than-standard condition is what leads to the altimeter reading being lower than the true altitude.

Temperature effects on altitude readings. The altimeter measures ambient pressure and converts it to an altitude. In warmer-than-standard air, the pressure falls off with height more slowly, so at a given true altitude the ambient pressure is higher than what the standard atmosphere would predict. With the altimeter set to the standard pressure lapse, this higher pressure makes the instrument indicate a lower altitude than you actually have. So the true altitude is higher than what the altimeter shows.

Wind doesn’t change the pressure the altimeter senses in level flight; it mainly affects ground track and groundspeed. That’s why wind isn’t the cause of a lower indicated altitude. The warmer-than-standard condition is what leads to the altimeter reading being lower than the true altitude.